This invention relates principally to the purification of 1,1,1,2-tetrafluoroethane, also designated R-134a, which has been of particular interest as a replacement for chlorofluorocarbons having similar physical properties, particularly R-12. R-134a may be prepared by reaction of other fluorocarbons, such as trichloroethylene or R-133a (2-chloro-1,1,1-tri fluoroethane) with HF. It may also be prepared by hydrogenation of R-114a (2,2-dichloro-1,1,1,2-tetrafluoroethane) or R-124 (2-chloro-1,1,1,2-tetrafluoroethane).
It is characteristic of such reactions that many by-products are formed, containing varying numbers of hydrogen, chlorine, and fluorine atoms on methane, ethane, and ethylene molecules. These by-products and the unreacted feed material may be separated by distillation where possible. Some compounds are relatively harmless since their presence does not greatly alter the physical properties for which HFC-134a is useful. A by-product which must be removed because of its toxicity is 2-chloro-1,1-difluoroethylene (R-1122), although only relatively small amounts are typically present in R-134a as formed. R-1122 has a boiling point close to that of R-134a making them difficult to separate by distillation. After distillation of the crude product, R-1122 will still be present in amounts from about 500 to 10,000 ppm (wt.). It should be reduced to below 10 ppm according to the specifications of the Panel for Advancement of Fluorocarbon Test (PAFTII). Preferably, the R-1122 should be below about 1 wt. ppm.
In U.S. Pat. No. 3,819,493 Fozzard discloses an extractive distillation process for separating 1,1-difluoroethane (R-152a) from R-134a produced by electrochemical fluorination of R-152a. The two compounds have a low relative volatility and saturated hydrocarbons having 4-10 carbon atoms are added to increase the relative volatility and facilitate their separation.
Bell in U.S. Pat. No. 4,129,603 removes R-1122 by contacting impure R-134a with an aqueous solution of a metal permanganate. The R-134a is derived from the reaction of HF with a haloethane such as 2-chloro-1,1,1-trifuloroethane over a chromium oxide or fluoride catalyst.
A different approach to removing R-1122 from R-134a is shown by Potter in U.S. Pat. No. 4,158,675. The reaction producing R-134a takes place at temperatures in the range of 325.degree. to 375.degree. C. in the presence of a chromium oxide or fluoride catalyst. Potter passes the effluent of the reaction into a second reactor containing a chromium catalyst but operated at 100.degree. to 275.degree. C. He shows that a substantial reduction of R-1122 is obtained.
In. U.S. Pat. No. 4,906,796 one of the present inventors disclosed the removal of R-1122 from R-134a by adsorption using zeolites or carbon molecular sieves.
Further improvement in methods of purifying R-134a, particularly with respect to eliminating R-1122 is desired and the present inventors have discovered a means for purification by photochlorination which will be disclosed in detail below.
In addition, one aspect of the invention relates to a process for production of R-124 (2-chloro-1,1,1,2-tetrafluoroethane).